1. Field of the Invention
The present invention relates to a method of manufacturing an electrolytic capacitor, and more particularly, it relates to a method of manufacturing an electrolytic capacitor by winding an anode foil and a cathode foil.
2. Description of the Background Art
At present, an electrolytic capacitor employing a conductive polymer such as polypyrrole, polythiophene, polyfuran or polyaniline or TCNQ complex (7,7,8,8-tetracyanoquinodimethane) as an electrolyte is watched with interest. A method of manufacturing such an electrolytic capacitor is now described. First, formation is performed on an aluminum foil, for forming an aluminum oxide film on the surface thereof. Then, the aluminum foil provided with the aluminum oxide film is cut into a prescribed size to form a zonal anode foil. Another aluminum foil is cut into a prescribed size to form a zonal cathode foil.
Then, an anode foil 103 and a cathode foil 104 cut into zonal shapes respectively are wound with separator sheets 105 and 106 interposed therebetween for forming a capacitor element 102, as shown in FIG. 5. An anode lead wire 109 electrically connected to anode foil 103 through a lead tab terminal 108 and a cathode lead wire 110 electrically connected to cathode foil 104 through a lead tab terminal 108 project from capacitor element 102 respectively.
Then, reformation (end face treatment) is performed on the cut surface (end face) of anode foil 103 exposing aluminum and a damaged portion of the aluminum oxide film. Then, capacitor element 102 is dipped in a prescribed chemical solution stored in a formation vessel 121, as shown in FIG. 6. Then, positive and negative potentials are applied to anode lead wire 109 and formation vessel 121 respectively, thereby forming films of aluminum oxide on the cut surface of anode foil 103 and the damaged portion of the aluminum oxide film.
Then, heat treatment is performed on capacitor element 102 at a temperature of about 150° C. to 300° C., to impregnate capacitor element 102 with a solid electrolyte. Then, capacitor element 102 impregnated with the solid electrolyte is sealed with a rubber packing 112 and stored in an aluminum case 111, as shown in FIG. 7. Then, an opening of aluminum case 111 is transversely drawn and curled. Thereafter aging is performed to complete an electrolytic capacitor 101. Japanese Patent Laying-Open No. 2000-277388 discloses a method of manufacturing this type of electrolytic capacitor, for example.
As hereinabove described, the aluminum oxide film is formed on the surface of the aluminum foil by formation in the anode foil of the conventional electrolytic capacitor. In recent years, increase in capacitance has been required to an electrolytic capacitor.
The inventor has attempted to apply a material having a higher dielectric constant to a film of an anode foil in order to increase the capacitance of a capacitor element without increasing the size thereof, and prototyped an electrolytic capacitor with an anode foil prepared from an aluminum foil provided with a dielectric film of niobium oxide, tantalum oxide or titanium oxide in place of aluminum oxide.
Thus, the inventor has proven for the first time that a capacitor element having such an anode foil is corroded or a deposit is formed on the surface of the capacitor element if the capacitor element is dipped in a formation vessel and subjected to reformation (see FIG. 6). In particular, it has been recognized that a portion of the capacitor element opposite to the side connected with a lead terminal is remarkably corroded.